Method for producing cellulose fibres

ABSTRACT

The invention relates to a process for the production of cellulosic fibres by extruding a solution of cellulose in a tertiary amine oxide through spinning holes of a spinning nozzle and conducting the extruded filaments into a precipitation bath via an air gap while they are drawn off, the filaments in the air gap being exposed to the flow of a gas. The invention is characterized in that for the temperature (T) of the gas before the contact with the filaments 60° C.&lt;T&lt;90° C. is correct.

[0001] The invention relates to a process according to the preamble of claim 1.

[0002] Such processes for the production of cellulosic fibres are known under the names of “amine-oxide process” or “Lyocell process”.

[0003] “Lyocell” is the generic name BISFA (The International Bureau for the Standardization of man-made fibres) has given to cellulose fibres which are produced by dissolving cellulose in an organic solvent without formation of a derivative and extruding fibres from that solution. By “an organic solvent”, a mixture of an organic chemical and water is understood therein. Such fibres are also known by the term of “solvent-spun fibres”. As an organic solvent, N-methyl-morpholino-N-oxide is used today on a commercial scale.

[0004] It is known that the properties of Lyocell fibres as well as the stability of the spinning process are substantially influenced by the conditions prevailing in the so-called air gap between the spinning nozzle and the precipitation-bath surface.

[0005] For example, it is known from PCT-WO 93/19230 to cool the extruded filaments immediately after the forming by means of a gas stream. In the following, this gas stream is referred to as “cooling air”. In the examples of PCT-WO 93/19230, the temperature of the cooling air is −6° C. to 24° C.

[0006] PCT-WO 94/28218 describes a similar process as does PCT-WO 93/19230. According to that document, the temperature of the cooling air is to be kept below 50° C.

[0007] From PCT-WO 95/02082 there is known a process according to which the spinning-hole diameter, the spinning-mass output per hole, the titer of the individual filament, the width of the air gap and the humidity of the air in the air gap are to be kept in certain ranges by a mathematical expression. In the examples of PCT-WO 95/02082, no information on the temperature of the cooling air is given. In the description there are generally indicated temperatures of between 10° C. and 60° C., preferably of between 20° C. and 40° C. PCT-WO 96/17118 deals with the humidity content of the cooling air. The highest cooling-air temperature indicated in that document is around 40° C.

[0008] According to PCT-WO 96/21758, the temperature of the cooling air may be 0° C. to 40° C. In PCT-WO 97/38153, the temperature of the cooling air is indicated as being −10° C. to 50° C. PCT-WO 98/58103 describes that in the case of a large number of extruded filaments, i.e., when spinning nozzles with a great number of spinning holes are used, a very humid climate in the air gap results. To ensure the stability of the spinning process also under those conditions, PCT-WO 98/58103 suggests that the spinning solution immediately before spinning should contain a specific proportion of cellulose and/or another polymer having a higher molecular weight.

[0009] One problem in spinning cellulose solutions in NMMO is that in the case of solutions having a high viscosity, the spinning solution has to be spun at increased temperatures. High viscosities of the spinning solution result for example when the cellulose concentration in the solution is high, which of course is desired from an economical point of view. High viscosities further result when pulps having high proportions of high-molecular cellulose are used.

[0010] However, the temperature of the spinning solution has to be kept high also when fibres having a smaller titer, e.g. smaller than 1 dtex, are to be spun. In the case of such fibres, the filaments in the air gap have to be stretched to a particularly high degree. Without increasing the temperature of the spinning solution, the viscosity of the spinning solution would also here be too high for this stretching.

[0011] Usually, during spinning, the temperature of the spinning solution should be 80° C. to 120° C., in particular 100° C. to 120° C. Since solutions of cellulose in NMMO are thermally unstable and tend to undergo exothermal decomposition reactions, it is, however, not desired to increase the temperature of the cellulose solution.

[0012] The present invention has as its object to provide a process according to the generic term, whereby cellulose solutions having a high viscosity can be spun better and fibres having small titers can be produced better.

[0013] This object is achieved in that for the temperature (T) of the cooling air before the contact with the filaments 60° C.<T<90° C. is correct.

[0014] Surprisingly, it has been shown that when using cooling air having higher temperatures in the range of claim 1, also higher-viscous cellulose solutions can be spun well, without having to increase the temperature of the spinning solution. Also fibres having small titers, e.g. 0.9 dtex, can be spun well without increasing the temperature of the spinning solution.

[0015] Furthermore, fibres which are produced using cooling air with higher temperatures have higher strength values than fibres which at the same temperature of the spinning solution are produced using cooling air with a lower temperature.

[0016] Preferably, the cooling air has a humidity content of 4 g of H₂O/kg of air up to 15 g of H₂O/kg of air.

[0017] In particular, the process according to the invention is suitable for the production of fibres having a titer of less than 1 dtex.

EXAMPLE 1

[0018] A spinning solution having 15% by weight cellulose (pulp: Cellunier F, manufacturer: Rayonnier), 10% by weight water and 75% by weight NMMO was spun into fibres while using cooling air with different temperatures.

[0019] The respective minimally obtainable titer of the fibres was measured: For this, the maximum drawing-off speed (m/min) of the fibres is determined by increasing the drawing-off speed up until the fibre breaks. This speed is noted and utilized for calculating the titer according to the calculation method described in PCT-WO 98/58103.

[0020] Further, the strength of the spun fibres in the conditioned state was determined, respectively. Temperature of the Strength Spinning Solution Temperature of the Minimum Titer Conditioned (° C.) Cooling Air (° C.) (dtex) (cN/tex) 100 20 2.01 38.1 100 50 1.70 38.7 100 60 1.59 40.1 100 70 1.36 39.8 100 80 1.32 40.6

[0021] From the table it is apparent that at temperatures of the cooling air of above 60° C., the minimally obtainable titer decreases considerably. Further, the strength of the fibres increases considerably.

EXAMPLE 2

[0022] A spinning solution having 14.6% by weight cellulose (pulp: Borregaard LVU), 9.5% by weight water and 75.9% by weight NMMO was spun in a continuous experimental plant into fibres having a titer of 1.3 dtex. At different temperatures of the cooling air used it was measured which spinning-mass temperature was necessary to be able to produce without disturbances fibres having that titer. Temperature of the Cooling Air (° C.) Spinning-mass Temperature Required (° C.) 22 116 65 109

[0023] From the table it is apparent that when using cooling air with a temperature of 65° C., it is possible to produce the fibres at a considerably lower temperature of the spinning solution.

EXAMPLE 3

[0024] A spinning solution having 15% by weight cellulose (pulp: Alicell VLV, manufacturer: Western Pulp), 10% by weight water and 75% by weight NMMO was spun into fibres while using cooling air with different temperatures. As described in Example 1, the minimally obtainable titer of the fibres as well as the strength of the spun fibres in the conditioned state were determined: Temperature of the Strength Spinning Solution Temperature of the Minimum Titer Conditioned (° C.) Cooling Air (° C.) (dtex) (cN/tex) 100 20 1.34 37.4 100 50 1.05 39.2 100 70 0.98 40.4 100 80 0.92 39.1

[0025] From the table it is apparent that when using cooling air with higher temperatures, it is well possible to produce fibres having a titer of less than 1 dtex. 

1. A process for the production of cellulosic fibres by extruding a solution of cellulose in a tertiary amine oxide through spinning holes of a spinning nozzle and conducting the extruded filaments into a precipitation bath via an air gap while they are drawn off, the filaments in the air gap being exposed to the flow of a gas, characterized in that for the temperature (T) of the gas before the contact with the filaments 60° C.<T<90° C. is correct.
 2. A process according to claim 1, characterized in that the gas is air.
 3. A process according to claim 2, characterized in that the flowing air has a humidity content of 4 g of H₂O/kg of air up to 15 g of H₂O/kg of air.
 4. A process according to anyone of claims 1 to 3, characterized in that filaments having a titer of less than 1 dtex are produced. 